In power transmission networks alternating current (AC) power is typically converted to direct current (DC) power for transmission via overhead lines and/or undersea cables. This conversion removes the need to compensate for the AC capacitive load effects imposed by the transmission line or cable, and thereby reduces the cost per kilometer of the lines and/or cables. Conversion from AC to DC thus becomes cost-effective when power needs to be transmitted over a long distance.
The conversion of AC to DC power is also utilized in power transmission networks where it is necessary to interconnect AC networks operating at different frequencies. In any such power transmission network, converters are required at each interface between AC and DC power to effect the required conversion, and one such form of converter is a voltage source converter (VSC).
A known voltage source converter is shown in FIG. 1 and includes a multilevel converter arrangement. The multilevel converter arrangement includes respective converter bridges 10 of cells 12 connected in series. Each converter cell 12 includes a pair of series-connected insulated gate bipolar transistors (IGBTs) 14 connected in parallel with a capacitor 16. The individual converter cells 12 are not switched simultaneously and the converter voltage steps are comparatively small, and so such an arrangement eliminates the problems associated with the direct switching of the series-connected IGBTs 14.
The capacitor 16 of each converter cell 12 must however have a high capacitive value in order to constrain the voltage variation at the capacitor terminals in such a multilevel converter arrangement. A DC side reactor 18 is also required in each converter bridge 10 to limit transient current flow between converter limbs 20, and thereby enable the parallel connection and operation of the converter limbs 20.
These factors lead to expensive, large and heavy equipment that has significant amounts of stored energy, which makes pre-assembly, testing and transportation of the equipment difficult.